1. Field of the Invention
The present invention relates to a backlight system, and particularly to a backlight system used for a liquid crystal display (LCD).
2. Description of Related Art
In an LCD, because liquid crystal is not a luminous element but is instead a control element regulating passage of light therethrough, a light source is needed. In most cases, the light source is combined with some other apparatus to become an integrated system that is called a backlight system or a front light system according to a direction of light emitted.
Referring to FIG. 6, a typical LCD assembly 100 comprises an LCD panel 101 and a backlight system 102. The backlight system 102 comprises a light source 1021, a light source cover 1022, a reflection plate 1023, a light guide plate 1024, and a reflection polarizer 1025. The light emitted from the light source 1021 enters the light guide plate 1024 directly, or indirectly by being reflected by the light source cover 1022. The reflection plate 1023 disposed under the light guide plate 1024 reflects light coming from the light guide plate 1024 back into the light guide plate 1024. Outgoing light from the light guide plate 1024 to the reflection polarizer 1025 is composed of different components of polarized light, i.e., P polarized light and S polarized light. The two polarized light components' polarization axes are perpendicular to each other. The S polarized light has proper polarization and passes through the reflection polarizer 1025, and is used to illuminate the LCD panel 101. The P polarized light is reflected by the reflection polarizer 1025 and reenters the light guide plate 1024. Reflected again by the reflection plate 1023, the P polarized light is turned into common light which can be seen as a combination of P polarized light and S polarized light as before, but just with a lower intensity. This common light is then directed to the reflection polarizer 1025 and is partly passed and partly reflected back. The process repeats again and again, so that the P polarized light reflected by the reflection polarizer 1025 is reused.
Although the P polarized light reflected by the reflection polarizer 1025 is reused, much of it is also lost in the process. This is because, when the P polarized light is reflected by the reflection polarizer 1025 to enter the light guide plate 1024, part of the P polarized light is absorbed by the light guide plate 1024. In addition, the residual light then also loses a little strength when it is reflected by the reflection plate 1023. In the whole recycling process, these losses cannot be ignored.
At the same time, the backlight system does not use a diffusing plate or other apparatus having a similar function to scatter the light. This causes an unevenness of the light emitted from the reflection polarizer 1025, so that illumination provided to the LCD panel 101 is not satisfactory.
Referring to FIG. 7, U.S. Pat. No. 6,448,955, issued Sep. 10, 2002, discloses an LCD assembly 200, which comprises a liquid crystal panel 201 and a backlight system 202. The liquid crystal panel 201 comprises a first plate 2011, a liquid crystal layer 2012, and a second plate 2013. The backlight system 202 comprises two light sources 2021, two corresponding light source covers 2022, two corresponding light guide plates 2024, a reflection plate 2023, a diffusion plate 2025, a brightness enhancing film 2026, a reflection polarizer 2027, and a special cover layer 2028.
In operation, the light emitted from the two light sources 2021 enters the two light guide plates 2024 directly, or indirectly after being reflected by the light source cover 2022. With the reflection plate 2023 disposed under the two light guide plates 2024, the outgoing light from the two light guide plates 2024 is directed to the diffusion plate 2025. Then the diffused light enters the brightness enhancing film 2026. The incident light at the reflection polarizer 2027 can be seen as a combination of two parts, i.e., the P polarized light and the S polarized light, and their polarization axes are perpendicular to each other. Having the proper polarization, the S polarized light passes through the reflection polarizer 2027 and is used to illuminate the LCD layer 2012; and the P polarized light is reflected by the reflection polarizer 2027 to enter the light guide plate 2024 through the brightness enhancing film 2026 and the diffusion plate 2025. Reflected by the reflection plate 2023, the P polarized light is turned into common light which can be seen as a combination of P polarized light and S polarized light, as before, but with a lower intensity. This common light is then directed to the reflection polarizer 2027, and is partly passed and partly reflected back. This process repeats again and again, so that the P polarized light reflected by the reflection polarizer 2027 is reused.
The addition of the diffusion plate 2025 makes the final light more uniform, and the use of the brightness enhancing film 2026 enhances the utilization efficiency of the light. However, there is still one problem unsolved. When the P polarized light is reflected by the reflection polarizer 2027 to enter the light guide plates 2024, part of the P polarized light is absorbed by the light guide plates 2024, and the residual light then loses a little intensity when it is reflected by the refection plate 2023. Given that there is a diffusion plate 2025 added to the system, in the whole process, the losses due to absorbance and reflection amount to a quite large proportion.
It is desirable to provide a backlight system and a light guide plate used therein which overcome the above problems.